Dervish

Hi all and Happy New Year!

I have a question, on start up (everytime) I get a puff of blue smoke from the LHS exhaust.

This never happened up until about 2 months ago when I had a compression test done and exhaust clamps replaced as they had corroded. The compression test was because I wanted to make sure my oil usage (1 litre/ 1000miles) was not due to any seal problems, all compression values are well within tolerance. I'm thinking the start of consistent blue smoke is just coincidence with the compression test...?

Anyway, I've got oil residue on the plenum body, mainly LHS. All pointing to faulty AOS/ VOS. Would this cause the blue smoke on start up, back pressure sucking oil back into chamber when stopping engine? There's NO smoke under load or WOT and clears after 1-2 minutes, it's really just 1 big puff, and 100% blue, not white.

Also has anyone replaced the stock AOS with the larger motorsport version, part number 996 107 926 00? Is it a straight swap, you don't need extra pipe work as Suncoast sell it with a pipe? I do aprox 5-10 track sessions a year...

Many thanks,

jrgordonsenior

Although the AOS primarily known for sending out clouds of white smoke, I've seen blue smoke too. With oil in your plenum I would just replace it with the $110. OEM unit. I'd be awfully surprised if that doesn't cure the issue.

The dual chamber Motosports AOS costs in the vicinty of $600 and is overkill for a street car driven occasionally at the track. It was originally designed for cars competeting in the Grand Am Koni series where they would occasionally experience hydro-lock costing them a new motor. IIRC it won't fit in the 3.4's....

Dervish

Thanks for the quick response.

Mine's a 3.6, I'm just thinking that I'm not doing the DIY (read up on it and it sounds a PITA) so if I'm going to pay the 5 hours labour or whatever it is I might as well pay extra for something which will outlast and outperform the stock when I need it to??? I will be doing Spa weekends, Nurburgring weekends, and plenty of DE days in 2011...

Macster

Remove the throttle body. Check for evidence of oil wetness on the TB and its butterfly valve. If there is any... AOS is highly suspect. Check the area where the hose from the AOS connects to the intake manifold. If any oil puddled/collected there.... AOS.

With the TB removed see what you can see in the manifold. If the manifold walls have signs of oil wetness... You know what.

Another check is with the engine running unscrew the oil filler tube cap and see if you can remove it. In some failure modes, the AOS causes the engine crankcase to run under extreme low pressure to the point the pressure difference makes removing the cap difficult even impossible (unless you're Superman).

If the AOS is original or a replacement and has some miles on it (50K? 60K? or more?) you might consider replacing it just in case.

BTW, the compression test while the numbers were good doesn't mean the oil rings are any good. Just that the compression rings and the cylinder bores and the head gaskets good. All nice to know to be sure.

Also, the compression test can't tell you if the valve stem or guides are worn or of the valve stems seals are leaking.

These can allow oil to enter the combustion chamber and make smoke.

The high oil consumption suggests that the oil control rings or the valve stems or guides or seals are passing some oil.

But eliminate the AOS first as a suspect.

Even if the oil rings or valve stems/guides or seals the source of the oil smoke you can run the engine a long long time before you need to take care of these items.

Sincerely,

Macster.

ivangene

yea, @ $7/quart it takes a lot of quarts to get up to $15k for a rebuild - my guess is rings and/or valve seals

Pac996

Could also be regular symptoms but cold weather gets you cranking longer with more fogged gas going out the exhaust after a labored start. How long is the crank and before the engine runs good? Cold weather and fuel injection used to be a curse. I hope its no biggy.

redridge

+1.... If it's coming from the aos, why would it smoke on the one side consistently. This will usually result with smoke with both exhaust.

ivangene

hate to ask....

what brand oil and how frequently is it changed?

also weight of oil

OZ951

G'day Stuart,

The fact that you get the smoke every time and only from the LHS makes me think its less likely but not impossible that the AOS is the culprit. I did have a bad AOS on my 996.2 and that would smoke every start at track days or after a spirited drive but it wouldn't smoke after 'normal' driving. Whenever it smoked it was always from both sides. A different failure mode could lead to different symptoms of course.

996sweden72

"1 litre/ 1000miles"

Is that normal? I would say that mine 996 2002 uses max 2l in 6000 miles, and I too drive my car hard..

Dervish

Thanks Guys.

Only slightly longer than normal but still starts fairly easily, no labored crank, engine runs 'clear' within 1-2 minutes. No other symptoms.

I'm using Silkolene ProS 5W-40 and I change at every DE or 5k miles, no problems with oil in that sense.

I read your reports on that and the fact that it smokes from 1 exhaust is slightly confusing, there is a very small puff from the RHS but much less than the LHS.

I usually take it for a spirited drive when ever I can and if I start her up within a couple of hours after then there is no smoke. It's only after it's been sitting for a day. Hence my thinking and going by Macsters explanation maybe it's during this longer sitting period that the oil mist in the inlets has time to 'drip' back into the chambers???

The oil on the plenum outside wall is definitely mainly on the LHS, could this be spray from the faulty AOS, a small crack in bellows??? I'll check the running with oil cap removed later today.

According to Porsche you can use up to 1litre/1000KM (or 621miles) so I'm within tolerance. This has been confirmed to me by Porsche.

The oil usage has stayed continuous since I bought her. She gets driven 'enthusiastically' with a proper warming up period. Oil gets changed very frequently (last year she had 5 oil changes).

Previously she might have smoked 3 times in 2 years (including after DE), now it's every time she's been sat for a day or longer and it suddenly started happening in the last 2 months. I'm just trying to figure out why before I put her in for a costly fault find...

Thanks again.

OZ951

Your description of the issue does sound more like valve stem seals (oil leaking past them after an over night sit). Sounds like a good question to ask Baz Hart with respect to how common the issue is. Two other tests that can indicate valve stem sealing issues are (1) to see if there is a puff of smoke after the car has been at idle for a couple of minutes and then you pull away (2) Let the car roll down a steepish hill in low gear (no throttle hence with the engine doing the braking & high vacuum) and then accelerate away at the bottom and see if there is a puff of smoke.

ivangene

it is possible that changing brands of oil might have some effect - we run a lot of oil threads here and on the 911 board where people find switching brands makes an unexpected change - sometimes for the better, sometimes for the worse...its just a thought

Dervish

I'll try those next weekend before taking it to the shop. In the meantime I'll try mailing/ calling Baz, problem is he's pretty far away from me.

I took the filler cap off whilst engine running and no problem there, easy to get cap off and mild change in engine running before settling.

I changed from Mobil1 to Silkolene after a lot of research and recommendations. Even going to a 10W-50 during the summer season as my oil pressure was 0.8-1bar at hot idle after back to back 'Ring laps on 5W-40. Last change was back to 5W-40 for colder starts and season end.

ivangene

good lord you are driving on the ring...


I would give you a few quarts if I can come

(what a dream that must be!)